1. Field of the Invention
The present invention relates to an electro-optic sampling oscilloscope, in which an electric field generated by a signal to be measured is coupled to an electro-optic crystal, an optical pulse is caused to enter this electro-optic crystal, and the waveform of the signal to be measured is observed by means of the polarization state of the optical pulse. In particular, the present invention relates to an electro-optic sampling oscilloscope that is provided with characteristic features in the generation of the optical pulse.
This application is based on patent application No.Hei 9-273157 filed in Japan, the content of which is incorporated herein by reference.
2. Background Art
It is possible to observe the waveform of a signal to be measured by coupling the electric field generated by the signals to be measured to an electro-optic crystal, causing laser light to enter this electro-optic crystal, and using the polarization state of the laser light. Here, it is possible to use this laser light in pulse form, and to conduct measurement with extremely high time resolution when the sampling of the signal to be measured is conducted. An electro-optic sampling oscilloscope employs an electro-optic probe which takes advantage of this phenomenon.
In comparison with conventional sampling oscilloscopes which employ electrical probes, such an electro-optic sampling oscilloscope (herein below termed an "EOS" oscilloscope) has the following characteristic features:
(1) When signals are measured, a ground wire is not required, so that measurement is simplified. PA0 (2) The metal pin which is at the lead end of the electro-optic probe is isolated from the circuit system, so that it is possible to realize a high input impedance, and as a result, the state at the point at which measurement is conducted is essentially free of fluctuations. PA0 (3) Since optical pulses are employed, measurement is possible in a broad band up to the order of GHz.
FIG. 5 serves to explain the measurement concept of the electro-optic probe in an EOS oscilloscope.
As shown in FIG. 5, a metal pin 21 is provided at the lead end of the electro-optic probe, and by placing this in contact with the signal line 31 which is the subject of measurement, an electric field 23 is generated based on the measured signal. In order to couple the electric field generated with an electro-optic crystal 22, the electro-optic crystal 22 is provided at the end of the metal pin 21. With respect to this electro-optic crystal 22, as a result of the Pockels effect, which is a primary electro-optical effect, the index of refraction of the electro-optic crystal changes in accordance with the coupled electric field strength, so that when an optical pulse 25 is inputted in this state, the polarization state of the optical pulse changes. The optical pulse 25 which experiences a change in polarization is reflected by reflection mirror 24, which is a multi-layered dielectric film mirror, and is guided to the light receiver 26, which serves as the input part of the polarization detecting optical system within the electro-optic probe (Shinagawa, et al: ""A High-Impedance Probe Based on Electo-Optic Sampling," Proceeding of the 15.sup.th Meeting on Lightwave Sensing Technology, May 1995, pp 123-129).
Next, the structure of the EOS oscilloscope will be explained using FIG. 6.
The EOS oscilloscope comprises an EOS oscilloscope main body 1 and an electro-optic probe 2. The optical pulse 25 explained in the discussion of FIG. 5 is generated in the optical pulse output circuit 4 on the basis of the trigger signal from a trigger circuit 3, and is supplied to the electro-optic probe 2. Additionally, once the optical pulse has experienced a change in polarization, the detection of the polarization thereof and the like are conducted by the polarization detecting optical system (not depicted in the figure) within the electro-optic probe 2, and the signal thereof is inputted into the EOS oscilloscope main body 1. Additionally, the amplification and A/D conversion of the signal are conducted by A/D circuit 5, and processing in order to display the signal which is the subject of measurement, and the like, is conducted by processing circuit 6.
FIG. 7 shows the details of the optical pulse output circuit 4 in FIG. 6. From FIG. 6, first, timing circuit 31 generates a timing signal, which is the optical pulse generating timing, based on the trigger signal from trigger circuit 3. Next, optical pulse generating circuit 32 generates a sample optical pulse based on the timing signal of timing circuit 31. Since the amount of light is insufficient with this sample optical pulse, the sample optical pulse is optically amplified by optical amplifier circuit 33, and the output of this optical amplifier circuit 33 is supplied to electro-optic probe 2 as an optical pulse.
When the sample optical pulse from the optical pulse generating circuit 32 is amplified by the optical amplifier circuit 33, ASE (Amplified Spontaneous Emitting), which is amplified spontaneously emitted light which is unnecessary and does not contribute to sampling in the spectrum or to sampling along the time axis, is generated.
FIG. 8 shows an example of the spectral analysis of output from the optical amplifier circuit 33 in the optical pulse output circuit 4 shown in FIG. 7. A sample optical pulse is outputted from the optical pulse generating circuit 32 which has essentially a single frequency, and light having other frequencies, which constitutes noise, is at a sufficiently low level that it can be ignored; however, light which is unnecessary and does not contribute to the sampling in the spectra is generated by optical amplifier circuit 33, so that light having frequencies other than the frequency outputted by the optical pulse generating circuit 32 is also outputted, and reaches a level at which it can not be ignored.
Furthermore, FIG. 9 shows an example of output from the optical amplifier circuit 33 with respect to time. It can be seen from FIG. 9 that, by means of optical amplifier circuit 33, light is also outputted by optical pulse generating circuit 32 at times other than those during which the sample optical pulse is generated, so that unnecessary light which does not contribute to the sampling along the time axis is produced.
In this way, while optical amplifier circuit 33 is necessary on the one hand in order to output an optical pulse having a sufficient amount of light, this contains unnecessary light which does not contribute to the sampling in the spectrum, and light is emitted by the optical pulse generating circuit 32 at times other than those during which the sample optical pulse is generated, so that as a result of such noise, the measurement accuracy of the EOS greatly decreases.